Semiconductor quantum well devices have been extensively studied and developed in recent years. They admit a high degree of applicability to photonic and optoelectronic circuits using tunable properties such as electro-refraction, electro-absorption, and the like. Lasers, modulators, detectors, bistable devices, and waveguides, all having quantum wells, have been designed and fabricated for such applications as optical computing, photonic switching, and optical communications.
Central to the operation of all the aforementioned quantum well devices is the fact that quantum well structures exhibit exciton resonances at room temperature. Exciton resonances are sharp absorption features near the optical band edge for the quantum well material. The existence of strong excitons which give rise to the absorption features is due to the confinement afforded by the quantum well region. Quantum wells are understood in practice as being a narrow energy bandgap material layer sandwiched between wider energy bandgap material layers wherein the thickness of the narrow energy bandgap material layer is much less than twice the exciton Bohr radius (&lt;&lt;2a.sub.0) for the narrow energy bandgap material. In GaAs, for example, a narrow energy bandgap material layer between two wider bandgap material layers such as AlGaAs is considered a quantum well when its thickness is less than 300 .ANG..
In certain quantum well devices, for example, the self electrooptic effect device (SEED) defined in U.S. Pat. No. 4,546,244, it has been noted the switching speed increases as the operating light intensity increases because the quantum well region of the SEED is essentially a capacitor and is therefore charging at a rate linearly proportional to the photocurrent. Build up of photoinduced carriers in the quantum wells caused by finite carrier escape times from the quantum wells limits intensity. Intensity is limited because the exciton in the quantum well saturates at a certain carrier level which causes a degradation of the electro-absorption characteristic of the quantum well. Also, the time required for carrier escape from the quantum well increases as the applied electric field decreases across the quantum well.
In order to overcome the carrier escape problem and, thereby, promote more rapid carrier escape from the quantum wells, it was suggested in 1990 that a reduction in barrier thickness and a decrease in barrier Al concentration, x, be used to improve the saturation performance for an Al.sub.x Ga.sub.1-x As/GaAs SEED. See Applied Physics Letters, Vol. 57, No. 22, pp. 2315-7 (1990). In the cited reference, the Al concentration was decreased from 0.3 to 0.2 to achieve what was stated to the most significant increase in saturation intensity. While such an apparently low Al concentration yielded a desirable decrease in carrier escape time from the quantum well, it is understood that the minimum Al concentration in the barrier layers was set at 0.2 in order to have a sufficient difference between the barrier height and the quantum well depth for maintaining exciton confinement in the quantum well layer. In a slightly earlier article in Applied Physics Letters, Vol. 54, No. 18, pp. 1716-8 (1989), it was stated that an Al concentration of 0.2 in an AlGaAs/GaAs quantum well structure created a "shallow well" which still maintained the necessary confinement to exhibit a coupled pair or exciton. In view of the statements in both articles, it can be surmised that sufficient exciton confinement for sharp absorption effects would not exist in quantum wells more shallow than those described above.